1. Field of the Invention
The present invention relates to the field of biosensors and is more specifically concerned with methods for providing metal surfaces with surface layers capable of selective biomolecular interactions. The invention also comprises activated surfaces for coupling a desired ligand; surfaces containing bound ligand; and the use of such surfaces in biosensors.
2. Description of Related Art
According to Aizawa (1983) a biosensor is defined as being a unique combination of a receptor for molecular recognition, for example a selective layer with immobilized antibodies, and a transducer for transmitting the interaction information to processable signals. One group of such biosensors will detect the change which is caused in the optical properties of a surface layer due to the interaction of the receptor with the surrounding medium. Among such techniques may be mentioned especially ellipsometry and surface plasmon resonance. In order for these types of techniques to work satisfactorily in actual practice certain requirements have to be fulfilled--i.e., the requirement that the sensing surface (or measuring surface) employed can easily be derivatized so that it will then contain the desired receptor, and moreover that it will not produce any (or only negligible) non-specific binding, i.e., binding of components other than those that are intended. In somewhat simplified terms the technique of surface plasmon resonance--by abbreviation SPR, as derived from the initials surface plasmon resonance may--may be said to be a technique in which changes in the refractive index in a layer close to a thin metal film are detected by consequential changes in the intensity of a reflected light beam (see for example Raether, H (1977).
Thus in this case, the sensing surface is a metal like film with receptors or "ligands" as they will be called henceforth, these being generally molecules or molecular structures which interact selectively with one or more biomolecules.
The metal film is applied on a substrate of a type that is suitable for the measuring method employed In the case of SPR, this means that a dielectric material, e.g., in the form of a glass plate, is used for directing a light beam to the metal surface.
According to most of the publications that have come forth up to now, SPR procedures when applied to detecting biomolecules have been carried out simply by adsorbing the biomolecule in question directly to the metal surface and then studying the consequential effect on the measuring signal. In a next step, this surface could optionally be used for binding a new layer of molecules (ligands) having an affinity for the first-bound layer of molecules. Thus for instance Liedberg, B. et al. (1983), in a first work indicating the potential of SPR technology for biochemical analyses, adsorbed at first a monolayer of IgG to a silver surface and then adsorbed an anti-IgG layer to said monolayer, in order to then study the effect with respect to the resultant change in the resonance angle.
Others too, e.g., Cullen DC et al. (1987/88), have utilized adsorption of biomolecules directly to a metal surface when studying immune complex formation in the IgG/anti-IgG system using the SPR technique with a gold-coated diffraction grating.
EP 257g55 describes a method according to which the metal film is coated with silica and optionally treated with a silanizing reagent; and in EP 202021 the metal film has been coated with an organic layer that may contain for example an antibody to a specific antigen Although the possibility of the antibody being bound covalently is indeed mentioned in that specification the actual nature of the organic layer is not disclosed or indicated at all, and the same applies to the manner in which the organic layer is produced.
According to EP 254575 an optical structure of the type such as is suitable for e.g. SPR applications may be produced by coating the metal film with a layer of an organic polymer, by means of the so-called "solvent casting technique" In a preferred embodiment cellulose nitrate is employed, and a number of well-known methods are mentioned for binding biospecific ligands to the layer.
Publications of this kind, while giving indications of the potential of the method, also demonstrate some of the limitations inherent in the technical solutions proposed.
As pointed out in for instance EP 254575, one of the problems is that biomolecules may be subject to an at least partial inactivation due to direct contact with metallic and certain inorganic surfaces. Another complication is that some ligands which may be desirable for some special applications cannot be adsorbed in a stable manner to a metal surface and thus cannot be expected to give reproducible results. Still another problem is that many of the media occurring in biochemical systems have a corrosive effect on the metal surface.
Although problems of these kinds may be solved at least in part by way of a process according to EP 254575, a construction of this type has a number of obvious drawbacks. A polymeric coating in the form of cellulose nitrate--as according to a preferred embodiment--will put a limit on the number of possible applications inasmuch as it is a well-known fact that biomolecules can be adsorbed irreversibly to cellulose nitrate films. In biosensor systems based on optical surface detection technology such a phenomenon may give rise to ambiguous and non-reproducible signals due to non-specific interaction between the sensing surface and components present in, for instance, human serum samples. Such side effects have been compensated for in EP 254575 by using a combination of a measuring and a reference surface. A requirement for the working of this method is that the nonspecific contribution is equally great on both surfaces; but this condition is not always fulfilled in actual practice.
Another problem is pointed out in EP 226470 regarding the production of constructs similar to the one mentioned above (see also U.S. Pat. No. 4415666). From the specification it can be seen how difficult it is to obtain acceptable stability, uniformity and reproducibility of the polymeric coating; and the consequential negative effects in cases where biosensor systems are employed will be readily appreciated.
Although for many practical uses a polymeric coating of the cellulose nitrate type having a thickness of 15-20 nm may indeed provide sufficient protection from corrosion, there is nevertheless an obvious risk that smaller molecules may penetrate through such a layer and cause an irreversible change in the metal surface. As shown below, sulfur compounds such as will be encountered in some situations associated with the present type of measurements--e.g., in cases where organic thiol compounds are used for reducing disulfide bonds in proteins--have a high affinity for noble metals, and upon being adsorbed will produce an uncontrolled alteration of the optical properties of the metals. It has also been shown that a polymeric coating of the cellulose nitrate type may be damaged by, e.g., detergent treatment with 2% SDS (see EP 254575).